The present disclosure relates generally to warning lights for use with emergency vehicles such as police cars, fire trucks and ambulances, and more particularly to an optical system for producing integrated directional light from a plurality of LED lamps.
Within any broad category of illumination devices such as warning lights, lights designed for a particular application, e.g., aircraft warning lights, may have a very different required intensity and light pattern than a warning light designed for, e.g., an emergency vehicle such as a police car or fire truck.
The prior art contains numerous examples of alternative light sources, reflectors and lenses arranged to produce particular intensities and distributions of light suited for a particular purpose. Of primary concern to designers of lights are efficiency and accuracy. By efficiency, it is meant that lighting designers are concerned with producing the maximum amount of light per unit of energy applied and transforming that light into a useful pattern with minimal losses. The light fixture must also distribute the available light as accurately as possible in the desired pattern. Any light that is scattered or not accurately directed in the desired pattern is effectively lost by being dispersed.
Light Emitting Diodes (LEDs), while efficient producers of light in terms of light produced per unit of applied energy, continue to be limited in terms of the quantity of light produced by individual LED lamps. Advances in LED technology have resulted in commercially available LED lamps with significantly improved light output. High-output (also referred to as high brightness) LEDs are now a practical light source for use in signaling and warning illumination. Even though high-output LED lamps have significantly greater light output (luminous flux) than previous LED lamps, the total luminous flux produced by each LED lamp is still relatively small, e.g., in the range of 50 to 130 lumens, and will typically have a very wide viewing angle of 110 to 160 degrees. Thus, these newer LED lamps produce a “half globe” of light in contrast to a directed beam of light. It is typically necessary to accumulate multiple LED lamps in a compact array and externally focus their light output to produce a light source with luminous intensity sufficient for many warning and signaling applications.
LED lamps are attractive to lighting designers because the light they produce is typically of a very narrow spectral wavelength, e.g., of a single pure color, such as red, blue, green, amber, etc. LED lamps are extremely efficient producers of colored light because the particular chemical compound used in the die of the LED, when excited by electrical current, produces a monochromatic band of energy within the visible light spectrum. For example, a red LED will generate a narrow wavelength of light in the visible red spectrum, e.g., 625 nm+/−20 nm. No external color filtering is needed, significantly improving the efficiency of the light source. Further, LED lamps are directional light sources. The light produced from an LED lamp is primarily directed along an optical axis through the center of the lens of the LED lamp. However, and in particular with the more recent high-output LED lamps, a significant portion of the light is also directed out the sides of the lens of the LED lamp (the above mentioned “half globe”). If the limited light output of an LED lamp is to result in a practical signaling or illuminating device, as much of the light produced by each LED lamp must be captured and directed in the desired light pattern as possible.
Various federal and state standards apply to warning light systems for fire, ambulance and police emergency response vehicles. Although there are differences among the standards, the basic requirement is that warning light systems provide a 360° pattern of visual warning around the vehicle. The standards are typically defined in terms of color and intensity, measured from particular vantage points around the vehicle. Many illumination standards measure intensity with respect to a horizontal plane passing through the center of the warning light in its installed orientation. Such a horizontal plane bisecting the warning light is a convenient reference for describing the structures of the warning light as well as the resultant photometric pattern. For warning purposes, the most effective photometric pattern for a surface-mount warning light is a vertically collimated wide-angle beam.
Another aspect of surface-mount warning lights relates to the aesthetic appearance of the warning light. Keeping the profile (profile is the extent to which the warning light protrudes from the vehicle body) of a surface-mount warning light as low as possible allows the warning light to blend with the surface of the vehicle to create an aesthetically pleasing and aerodynamic appearance. Another aesthetic objective of warning light design is to provide a uniformly illuminated surface area with minimal “dead” areas rather than points of intense illumination surrounded by reduced intensity or dark areas.